Structural Stability of Graphene Nanoflakes

Jun Nakamura (ee.uec.ac.jp), Universidad de Electrocomunicaciones (Tokio), Japón.

The so-called Clar formula is useful for describing the resonance structure of polycyclic aromatic hydrocarbons (PAHs) in terms of aromaticity. As for doped systems, however, it is not always valid to predict the aromatic rings using the Clar formula, since the number of ff electrons for dopants and neighboring carbon atoms is not necessarily predictable unlike in the case of non-doped carbon systems. We have quantitatively investigated the relationship between the aromaticity and structural stability of graphene nanoflakes (GNFs) using first -principles calculations.

The aromaticity of each sixmembered ring of GNFs is evaluated with the nucleus-independent chemical shifts (NICS). We have found that for armchair-edge GNFs, the degree of stability is proportional to the average NICS for all six-membered rings. Even for doped GNFs, the average NICS strongly correlates with the structural stability. Our results indicate that NICS is a good measure not only for the aromaticity but also for the structural stability of pristine/doped nanographene systems. It has also been clarified that aromaticity is relevant to stability not only for pristine GNFs but also for doped GNFs, where the Clar formula is necessarily applicable.

Jun Nakamura (@junj_capriccio) en el Congreso Nacional de la Sociedad Mexicana de Física: https://t.co/OhmkP1Ksqe #physics pic.twitter.com/h2mYHRkHNQ

— Ernesto Mata Plata (@elmapa_cc) March 31, 2020

Conclusions

We have examined the stabilities of pristine GNFs and doped GNFs and related them to their aromaticity by considering NICS. The stability of non-doped GNFs is dominated by the following factors: The formation of zigzag edges make GNFs unstable because of the existence of edge states. The average NICS strongly correlates with structural stability regardless of the shape of GNFs. For both nitrogen- and boron-doped GNFs, the dopant atom is preferably doped near zigzag edges. This is because the electron=hole from the itrogen=boron atom migrates to the unoccupied=occupied edge states.

Even for doped GNFs, the average NICS positively correlates with the doping formation energy, whereas the six-membered rings around the dopant become antiaromatic. The Clar formula is useful for describing the resonance structure of PAHs in terms of aromaticity. As for doped systems, however, it is not always valid to predict the aromatic rings using the Clar formula, since the number of π electrons for dopants and neighboring carbon atoms is not necessarily predictable unlike in the case of non-doped carbon systems. In spite of the lack of the Clar structure, for nitrogen- and boron-doped ZZGNF systems, NICS is certainly a good measure of structural stability. It has been clarified that aromaticity is relevant to stability not only for pristine GNFs but also for doped GNFs, where the Clar formula is necessarily applicable.